We quantify the greenhouse-gas mitigation potential and carbon abatement costs if green waste in the metropolitan region of Berlin, Germany, is diverted from composting into the production of hydrothermally carbonized coal (HTC coal) that is used to substitute for hard coal in electricity and heat generation. Depending on the origin of the green waste, we specify an urban, a rural-urban, and a rural scenario. All scenarios combined can mitigate 70,511 metric tons (t) of carbon-dioxide equivalents (CO2e) per year. The carbon abatement costs reach 163 €/t CO2e in the urban scenario, 76 €/t CO2e in the rural-urban scenario, and 77 €/t CO2e in the rural scenario. The lower abatement costs in the latter two scenarios are mainly due to HTC-coal co-firing in an existing power plant rather than constructing a new biomass power plant for HTC-coal mono-firing as in the urban scenario. While the abatement costs exceed the current carbon prices, they compare more favorably with commonly assumed damage costs of unmitigated climate change. Thus, the public support of HTC coal could be considered, with the primary policy focus on HTC-coal co-firing. HTC-coal co-firing could also lower the emissions of existing power plants during the fossil-fuel phase-out.